1. Field of the Invention
The present invention relates to an objective lens driving apparatus.
2. Description of the Related Art
An objective lens driving apparatus is known which drives an objective lens for focusing a laser beam for recording information on a signal recording surface of an optical disc or reproducing information therefrom. At the time of the recording or reproduction, the laser beam needs to follow a track and to be focused thereon. Therefore, the objective lens driving apparatus is capable of moving the objective lens in a focusing direction perpendicular to a disc surface of the optical disc and a tracking direction perpendicular to the track on the disc surface (radial direction of the optical disc), so that focus deviation and tracking deviation of the laser beam are corrected (focusing control and tracking control). The objective lens driving apparatus is a part of an optical pickup apparatus that records information into an optical disc or reproduces information therefrom, for example.
An objective lens driving apparatus having two axes is known which is capable of driving an objective lens in the focusing direction and the tracking direction (see, e.g., Japanese Patent Application Laid-Open Publication No. 2006-260704).
An exemplary configuration of an objective lens driving apparatus 91 will hereinafter be described with reference to FIGS. 6A and 6B. FIG. 6A is a plan view of an objective lens holder 910 and magnets 96 included in the objective lens driving apparatus 91, and FIG. 6B is a side view of the objective lens holder 910 and the magnets 96.
The objective lens driving apparatus 91 exemplarily illustrated in FIGS. 6A and 6B includes the objective lens holder 910 in a substantially rectangular parallelepiped shape elongated in the tracking direction. The objective lens holder 910, for example, holds an objective lens OBL centrally in the tracking direction on a main body in the substantially rectangular parallelepiped shape, includes a total of two focusing coils 920 respectively provided at both ends in the tracking direction of the main body, and includes a total of four tracking coils 930 respectively provided on both sides of the both ends of the main body. Each focusing coil 920 has a winding axis in the focusing direction, and each tracking coil 930 has a winding axis in a tangential direction (tangential direction of the track). Although the objective lens holder 910 is supported by a predetermined supporting means on a predetermined base, the base, the supporting means, etc., are not depicted for convenience of illustration.
As shown in FIGS. 6A and 6B, for example, two yokes 95 are projected from the base (not shown) so as to be respectively surrounded by the two focusing coils 920. Each of the four monopole magnets 96 is fixed to the base so as to be opposed to a portion of each of the four respective tracking coils 930 in the tracking direction. In FIGS. 6A and 6B, all surfaces of the magnets 96 opposed to the objective lens holder 910 are magnetized into N-poles, for example.
In the case of the focusing control, if currents are supplied to the focusing coils 920 in a counterclockwise direction as shown by arrows in FIG. 6A, a force in the focusing direction on a positive side (on a side of an optical disc not shown mounted on a turn table not shown) acts on the focusing coils 920 in a magnetic field between the magnets 96 and the yokes 95, so that the objective lens holder 910 is moved in the same direction on the same side.
In the case of the tracking control, if currents are supplied to the tracking coils 930 in the direction shown by arrows in FIGS. 6A and 6B, for example, a force in the tracking direction on a positive side acts on the tracking coils 930 in the magnetic field between the magnets 96 and the yokes 95, so that the objective lens holder 910 is moved in the same direction on the same side.
As shown in FIG. 6B, the tracking coils 930 and the magnets 96 are so arranged as not to be opposed completely with respect to the tracking direction so that an effective force for the tracking control acts on the tracking coils 930. In the magnetic field between the magnets 96 and the yokes 95, a force F1 in the tracking direction acts on the current flowing through one side b of a pair of sides a and b making up the tracking coil 930 in a substantially rectangle shape, for example. However, if the other side a is also in the same magnetic field, a force in a direction opposite to F1 in the tracking direction acts on the current flowing through the side a, and therefore, the two forces consequently cancel each other out. To avoid this situation, an arrangement configuration is made such that the side a is not opposed to the magnet 96 so that mainly F1 acts on the tracking coil 930 in the tracking direction. Since almost no force acts on the current flowing through the side a of the tracking coil 930 as above, it cannot be said that the current flowing through the tracking coil 930 is sufficiently effectively utilized for the tracking control, and therefore, it become difficult to improve sensitivity of the tracking control. This may result in insufficient improvement in the recording/reproducing characteristics of the optical pickup apparatus.
Description will be made of the case of further performing the tracking control in a state where a portion of the tracking coil 930 lies off the magnet 96 by supplying a current to the focusing coil 920 to move the objective lens holder 910 in the positive focusing direction, as shown in FIG. 6B. Firstly, with regard to a pair of sides c and d making up the tracking coil 930 on the positive side in the tracking direction, although a force F2 in the positive focusing direction acts on the current flowing through one side d opposed to the magnet 96, the other side c is not opposed to the magnet 96, and therefore, almost no force acts on the current flowing through the side c. On the other hand, with regard to a pair of sides c and d making up the tracking coil 930 on the negative side in the tracking direction, although the force F2 in the focusing direction on the negative side (on a side away from the optical disc) acts on the current flowing through one side d opposed to the magnet 96, the other side c is not opposed to the magnet 96, and therefore, almost no force acts on the current flowing through the side c. As a result, the force F2 in a direction toward the optical disc acts on the two tracking coils 930 on the positive side in the tracking direction while the force F2 in a direction away from the optical disc acts on the two tracking coils on the negative side in the tracking direction, and thus, counterclockwise moment in FIG. 6B acts on the objective lens holder 910. If the objective lens holder 910 rolls by the action of such moment, coma aberration is generated in the objective lens OBL and the recording/reproducing characteristics of the optical pickup apparatus may be deteriorated. The above phenomenon in which a portion of the tracking coil 930 lies off the magnet 96 tends to be more marked, as the optical pickup apparatus becomes thinner in the focusing direction. This leads to a conflict between the thinning down and the maintenance or improvement in recording/reproducing characteristics, in specifications of the optical pickup apparatus.
As shown in FIG. 6A, a pair of the magnets 96 in the tangential direction is arranged so as not to completely sandwich the focusing coils 920 with respect to the tracking direction, due to the relationship with the tracking coils 930. In the magnetic field between the magnets 96 and the yokes 95, a force in the focusing direction acts on the current flowing through portions e sandwiched by the magnets 96 in a pair of sides making up the focusing coil 920 in a substantially rectangular shape, however, almost no force acts on the current flowing through portions f not sandwiched by the magnets 96. On the other hand, it is known that antinodes of a standing wave in high-order bending vibration mode specific to the objective lens holder 910 is generally positioned at the both ends in the tracking direction of the objective lens holder 910. Therefore, if forces in the focusing direction on a predetermined side do not act on portions of the above f closer to the both ends, it becomes difficult to restrain high-order resonance excited at the time of the focusing control. In this case, the recording/reproducing characteristics of the optical pickup apparatus may be deteriorated due to the high-order resonance of the objective lens OBL.